This invention relates to a crankcase chamber precompression type of two-cycle internal combustion engine and more particularly to an improved scavenging port arrangement for such engines.
As is well known, two-cycle engines have considerable desirability, primarily due to their compact and uncomplicated structure. In addition, the fact that each cylinder fires on each revolution permits a greater power output to be obtained with a two-cycle engine of a given displacement than a four-cycle engine of the same displacement. However, the use of porting in order to admit and discharge the gases to the combustion chambers of the engine have some disadvantages. For example, it is necessary to ensure a full charge of fresh gases and fuel for each combustion cycle. However, at the same time, it is also necessary to ensure that substantially all of the burned exhaust gases from the previous cycle are discharged when the fresh charge is introduced.
In order to provide complete charging and scavenging, it has been the practice to employ multiple scavenge and/or exhaust ports. Frequently, the use of such multiple ports positions adjacent scavenge and exhaust ports in close proximity to each other. When this occurs, there is a risk that some of the fresh scavenge charge entering the cylinder through the scavenge ports adjacent the exhaust ports may flow directly out of the exhaust port with loss of fuel efficiency and scavenging efficiency. It has, therefore, been proposed to provide some form of masking for the side edges of the scavenge ports that are adjacent the exhaust ports to preclude such escape of the scavenge charge.
One way in which the side edges of the scavenge ports have been masked is through the use of a cylinder liner and cylinder block in which the liner ports do not completely mate with the cylinder block passage openings. In fact, it has been the practice to provide a somewhat larger opening in the liner than in the cylinder block so as to ensure that manufacturing errors do not cause undesired masking of the various ports.
The aforenoted concept has been employed so as to provide some masking of the side edges of the scavenge ports adjacent the exhaust port. That is, the side edge of the scavenge port can be masked by having the port in the cylinder liner displaced relative to the port opening in the cylinder block so that the side of the scavenge passage is masked by the offset of the cylinder liner edge from the cylinder block edge which define the respective openings.
Although the aforenoted construction is very advantageous in avoiding loss of scavenge charge out of the exhaust port, there still is a problem with escape of scavenge charge from the exhaust port. Specifically, as the piston moves upwardly toward its top dead center position, the scavenge charge may flow from the upper portion of the scavenge port adjacent the exhaust port around and exit from the exhaust port. This is likely because at the end of the scavenge stroke the scavenge gases are entering the cylinder at a relatively low velocity and can easily escape through the adjacent exhaust port. The aforenoted masking does not completely solve this problem.
It is, therefore, a principal object of this invention to provide an improved scavenging port arrangement for a two-cycle engine.
It is a further object of this invention to provide an improved arrangement for the scavenging port of an internal combustion engine to ensure that no scavenge charge flows out of an adjacent exhaust port even if the scavenge and exhaust ports are disposed closely adjacent to each other.